The subject matter disclosed herein relates to heating, ventilation and air conditioning (HVAC) systems. More specifically, the subject matter disclosed herein relates to falling film evaporators for HVAC systems.
In falling film evaporators, saturated two-phase refrigerant is distributed over an evaporator tube bundle both in an axial direction along a length of the tube bundle and a lateral direction over a width of the tube bundle. Poor or uneven distribution results in reduced efficiency of the evaporator, which is compensated for by utilizing larger evaporators.
Two-phase flow distribution inside evaporators is challenging. Liquid and vapor in a saturated mixture have substantially different enthalpies and tend to separate due the density difference between the two phases making even distribution difficult. A typical approach to alleviate this issue involves separating the liquid and vapor refrigerant in a separator upstream of the evaporator distributor, either internal to the evaporator or outside of the evaporator. The vapor is routed back to the compressor, while the liquid refrigerant is distributed over the tube bundle via gravity, flowing the liquid refrigerant through ports in a distribution plate located over the tube bundle. While separation of vapor and liquid refrigerant increases the uniformity of liquid refrigerant distribution over the tube bundle, for uniform distribution, the orifice area in the distribution plate must be small enough such that liquid covers the plate and a liquid seal over the ports is achieved at minimum load. Otherwise substantial maldistribution can occur. This creates an issue with contaminates plugging the small ports. Larger but fewer ports can result in poor coverage of liquid over the tubes. Additionally, the flow through the ports is controlled by the hydrostatic head over the plate and at full load the liquid height must increase substantially in order to satisfy the higher flow rate demand through the ports. This results in very large distributors and a large refrigerant volume.